evil twin

a character in a work of fiction who is the evil identical twin, clone, or counterpart of another character: The actor plays two roles—the protagonist and his evil twin, who serves as an antagonist for much of the film.

any person or thing that is the negative counterpart of another person or thing: You may find yourself haunted by hope's evil twin, regret, wondering why you ever bought the stock in the first place.

a hidden wireless internet access point operating near a legitimate one, but which is actually used by cybercriminals to divert sensitive information such as account numbers and passwords